Within a period of 15 months 8 cases of invasive pulmonary aspergillosis were seen in a haematologic unit; 7 of them between January and October 1984. No previous cases of invasive aspergillosis had been encountered during the existence of the unit since 1979. Environmental studies did not prove a single likely source for the fungal spores. Previous window renovation with concomitant fiber deposits on ventilation grids, poor sealing of air filter fittings in patient rooms, occasional ventilation through windows in the ward, and low speed of booster fans in the ventilation system may have created a condition favourable for the entry of Aspergillus fumigatus spores into the patient rooms. Environmental sanitation including cleaning of the ventilation ducts and change of filters in the ventilation system stopped the outbreak. Two sporadic cases have appeared during a follow-up period of 26 months.